The fourth and last class, and at the same time the most highly developed class of Molluscs, is that of the Cuttles, or Poulps, also called Cephalopoda (foot attached to the head). They all live in the sea, and are distinguished from Snails by eight, ten, or more long arms, which surround the mouth in a circle. The Cuttles existing in our recent oceans—the Sepia, Calamary, Argonaut, and Pearly Nautilus—are, like the few Spiral-gill Lamp-shells of the present time, but a poor remnant of the host which represents this class in the oceans of the primordial, primary, and secondary periods. The numerous fossil “Ammon’s horns” (Ammonites), “pearl boats” (Nautilus), and “thunderbolts” (Belemnites) are evidences of the long since extinct splendour of the tribe. The Poulps, or Cuttles, have probably developed out of a low branch of the snail class, out of the Butterfly-snails (Pteropoda) or kindred forms.
The different sub-classes and orders, distinguished in the four classes of Molluscs, whose systematic succession is given on the Table (p. [160]), furnish various proofs of the validity of the law of progress by their historical development and by the systematic development corresponding to it. As however these subordinate groups of Molluscs are in themselves of no further special interest, I must refer to the sketch of their pedigree on p. [161], and to the detailed pedigree of Molluscs which I have given in my General Morphology, and I shall now at once turn to the consideration of the tribe of Star-fishes.
The Star-fishes (Echinoderma, or Estrellæ) among which are the four classes of Sea-stars, Sea-lilies, Sea-urchins, and Sea-cucumbers are one of the most interesting divisions of the animal kingdom, and yet we know less about them than about any. They all live in the sea. Every one who has been at the sea shore must have seen at least two of their forms, the Sea-stars and the Sea-urchins. The tribe of Star-fishes must be considered as a completely independent tribe of the animal kingdom on account of its very peculiar organization, and must be carefully distinguished from the Animal-plants—Zoophytes, or Cœlenterata, with which it is still frequently but erroneously classed under the name Radiata (as for example, by Agassiz, who even to this day defends this error of Cuvier’s, together with many others).
All Echinoderma are characterized, and at the same time distinguished from all other animals, by a very remarkable apparatus for locomotion, which consists of a complicated system of canals or tubes, filled with sea water from without. The sea water in these aqueducts is moved partly by the strokes of the cilia, or vibratile hairs lining their walls, and partly by the contractions of the muscular walls of the tubes themselves, which resemble india-rubber bags. The water is pressed from the tubes into a number of little hollow feet, which thereby become widely distended, and are then employed for walking and suction. The Sea-stars are moreover characterized by a peculiar calcareous formation in the skin, which in most cases forms a firm, well-closed coat of mail, composed of a number of plates. In almost all Echinoderma the body consists of five radii (counterparts, or antimera) standing round the main axis of the body, where they meet. It is only in some species of Sea-stars that the number of these radii amount to more than five—to 6-9, 10-12, or even to 20-40; and in this case the number of radii is generally not constant, but varies in different individuals of one species.
The historical development and the pedigree of the Echinoderma are completely revealed to us by their numerous and, in most cases, excellently preserved fossil remains, by their very remarkable individual developmental history, and by their interesting comparative anatomy; this is the case with no other tribe of animals, even the Vertebrata themselves are not to be excepted. By a critical use of those three archives, and by a careful comparison of the results derived from their study, we obtain the following genealogy of the Star-fishes, which I have already published in my General Morphology (vol. ii. Plate IV. pp. 62-77.)
The most ancient and original group of the Star-fishes, the primary form of the whole phylum, consists of the class of the true Sea-stars (Asterida). This is established by numerous and important arguments in anatomy and the history of development, but above all by the irregular and varying number of the radii, or antimera, which in all other Echinoderma is limited, without exception, to five. Every Star-fish consists of a central, small, body-disc, all round the circumference of which are attached five or several long articulated arms. Each arm of the Star-fish essentially corresponds in its organisation with an articulated worm of the class of Ring-worms, or Annelida (p. [149]). I therefore consider the Star-fish as a genuine stock or cormus of five or more articulated worms, which have arisen by the star-wise growth of a number of buds out of a central mother-worm. The connected members, thus grouped like the rays of a star, have inherited from the mother-worm the common opening of the mouth, and the common digestive cavity (stomach) lying in the central body-disc. The end by which they have grown together, and which fuses in the common central disc, probably corresponds to the posterior end of the original independent worms.
In exactly the same way several individuals of certain kinds of worms are united so as to form a star-like cormus. This is the case in the Botryllidæ, compound Ascidians, belonging to the class of the Tunicata. Here also the posterior ends of the individual worms have grown together, and have formed a common outlet for discharges, a central cloaca; whereas at the anterior end each worm still possesses its own mouth. In Star-fishes the original mouths have probably become closed in the course of the historical development of the cormus, or colony, whereas the cloaca has developed into a common mouth for the whole cormus.
| SYSTEMATIC SURVEY | ||||||||
| Of the 4 Classes, 9 Sub-classes, and 20 Orders of Star-fishes. | ||||||||
| (Compare Gen. Morph. II. Plate IV. pp. 62-67.) | ||||||||
| Classes of Star-fishes. | Sub-classes of Star-fishes. | Orders of Star-fishes. | Systematic Name of the Orders. | |||||
| I. Sea Stars Asterida |
| I. Sea Stars with radiated stomach Actinogastra |
| 1. | Primary Stars | 1. | Tecastra | |
| 2. | Articulated Stars | 2. | Colastra | |||||
| 3. | Brisinga Stars | 3. | Brisingastra | |||||
| II. Sea Stars with disc-shaped stomach Discogastra |
| 4. | Serpent Stars | 4. | Ophiastra | |||
| 5. | Tree Stars | 5. | Phytastra | |||||
| 6. | Lily Stars | 6. | Crinastra | |||||
| II. Sea Lilies Crinoida |
| III.Lilies with arms Brachiata |
| 7. | Plated Lilies with arms | 7. | Phatnocrinida | |
| 8. | Articulated Lilies with arms | 8. | Colocrinida | |||||
| IV. Lilies with buds Blastoidea |
| 9. | Regularly budding Lilies | 9. | Pentremitida | |||
| 10. | Lilies budding on two sides | 10. | Eleutherocrina | |||||
| V. Bladder Lilies Cystidea |
| 11. | Bladder Lilies without stalks | 11. | Agelacrinida | |||
| 12. | Bladder Lilies with stalks | 12. | Sphæronitida | |||||
| III. Sea Urchins Echinida |
| VI. Older Sea Urchins (with more than 20 rows of plates) Palechinida |
| 13. | Palechinida with more than 10 rows of ambulacral plates | 13. | Melonitida | |
| 14. | Palechinida with 10 rows of ambulacral plates | 14. | Eocidaria | |||||
| VII. More recent Sea Urchins (with 20 rows of plates) Autechinida |
| 15. | Autechinida with band-like ambulacra | 15. | Desmosticha | |||
| 16. | Autechnidia with branching tentacles | 16. | Petalosticha | |||||
IV. Sea Cucumbers Holothuriæ |
| VIII. Sea Cucumbers with aquatic feet Eupodia |
| 17. | Eupodia with scutiform tentacles | 17. | Aspidochirota | |
| 18. | Eupodia with branching tentacles | 18. | Dendrochirota | |||||
| IX. Sea Cucumbers without aquatic feet Apodia |
| 19. | Apodia with water-lungs | 19. | Liodermatida | |||
| 20. | Apodia without water-lungs | 20. | Synaptida | |||||
